In an elevator system, there must be a safety system, by the aid of which the operation of the elevator system can be stopped e.g. as a consequence of a defect or of an operating error. The aforementioned safety system comprises a safety circuit, which comprises safety switches connected to each other in series, which switches are disposed in different parts of the elevator system to monitor the safety of the system. The safety switches are connected to each other in series with conductors, in which an electrical safety circuit signal travels as long as the safety switches are closed. When a safety switch opens, the passage of the safety circuit signal in the safety circuit is disconnected. The opening of a safety switch indicates that the safety of the elevator system has been jeopardized. The safety circuit signal can be e.g. a direct-current signal or an alternating-current signal of the voltage frequency of the electricity network.
The length of the safety circuit often forms a problem, particularly in high-rise buildings, because the resistance in long conductors causes a large voltage loss in the safety circuit signal. In addition, the loop of the safety circuit traveling in particular in the traveling cable of the elevator car is very sensitive to interference, because there are also many other signals, sometimes for a distance of hundreds of meters, traveling in the immediate proximity of the safety circuit conductors in the traveling cable.
A breakage of the protective sheath of a safety circuit conductor might cause an earth fault, in which case an earth fault current starts to travel from the electricity supply of the safety circuit into a conducting structure in the building. As a result of the earth fault the operation of the elevator must be stopped; on the other hand, an earth fault current might also cause failure of one or more elevator components.